X-ray imaging has been used in a variety of fields for imaging objects. For example, X-ray imaging has been used extensively in the medical field for non-destructive testing and X-ray computed tomography (CT). Various other types of technology are also being used for medical imaging. For example, diffraction enhanced imaging (DEI) is an X-ray imaging technique that dramatically extends the capability of conventional X-ray imaging.
The DEI technique is an X-ray imaging modality capable of generating contrast from X-ray absorption, X-ray refraction, and ultra-small angle scatter rejection (extinction). In contrast, conventional X-ray imaging techniques measure only X-ray attenuation. The DEI absorption image and peak image show similar information to a conventional radiograph, except that it is virtually free of scatter degradation. DEI utilizes perfect crystal diffraction from to convert small angular changes in a transmitted x-ray beam into large intensity changes in an image. Thus, DEI is well suited to soft-tissue imaging, and very promising for mammography.
The use of a silicon analyzer crystal in the path of the X-ray beam generates two additional forms of image contrast, X-ray refraction, and extinction (ultra small angle scatter rejection). DEI utilizes highly collimated X-rays prepared by X-ray diffraction from perfect single-crystal silicon. These collimated X-rays are of single X-ray energy, practically monochromatic, and are used as the beam to image an object.
Objects that have very little absorption contrast may have considerable refraction and extinction contrast, thus improving visualization and extending the utility of X-ray imaging. Applications of DEI techniques to biology and materials science have generated significant gains in both contrast and resolution, indicating the potential for use in mainstream medical imaging. An area of medicine where DEI may be particularly effective is in breast imaging for cancer diagnosis, where the diagnostic structures of interest often have low absorption contrast, making them difficult to see. Structures with low absorption contrast, such as the spiculations extending from a malignant mass, have high refraction and ultra-small angle scatter contrast. It is desirable to provide a DEI system with the capability to increase both the sensitivity and specificity of X-ray-based breast imaging.
Multiple studies have demonstrated improved image contrast in both medical and industrial applications of DEI. Advantages of DEI systems over conventional X-ray imaging systems in the medical field include a dramatic reduction in patient radiation dose and improved image quality. The dose reduction is due to the ability of DEI systems to function at higher X-ray energies. X-ray absorption is governed by the photoelectric effect, Z2/E3, where Z is the atomic number and E is the photon energy.
A monoenergetic radiograph contains several components that can affect image contrast and resolution: a coherently scattered component Ic, an incoherently scattered component Ii, and a transmitted component. X-rays passing through an object or medium where there are variations in density can be refracted, resulting in an angular deviation. Specifically, deviations in the X-ray range result from variations in ρt along the path of the beam, where ρ is the density and t is the thickness. A fraction of the incident photons may also be diffracted by structures within an object, which are generally on the order of milliradians and referred to as small angle scattering. The sum total of these interactions contributed to the recorded intensity in a radiograph IN, which can be represented by the following equation:IN=IR+ID+IC+II System spatial resolution and contrast will be degraded by the contributions of both coherent and incoherent scatter. Anti-scatter grids are often used in medical imaging to reduce the contribution of scatter, but their performance is limited and use of a grid often requires a higher dose to compensate for the loss in intensity.
The DEI technique utilizes a silicon analyzer crystal in the path of the post-object X-ray beam to virtually eliminate the effects of both coherent and incoherent scatter. The narrow angular acceptance window of the silicon analyzer crystal is referred to as its rocking curve, and is on the order of microradians for the X-ray energies used in DEI. The analyzer acts as an exquisitely sensitive angular filter, which can be used to measure both refraction and extinction contrast. Extinction contrast is defined as the loss of intensity from the incident beam due to scattering, which can produce substantial improvements in both contrast and resolution.
The Darwin Width (DW) is used to describe reflectivity curves, and is approximately the Full Width at Half Maximum (FWHM) of the reflectivity curve. Points at −½ DW and +½ DW are points on the curve with a steep slope, producing the greatest change in photon intensity per microradian for a particular analyzer reflection and beam energy. Contrast at the peak of the analyzer crystal rocking curve is dominated by X-ray absorption and extinction, resulting in near scatter-free radiographs. Refraction contrast is highest where the slope of the rocking curve is greatest, at the −½ and +½ DW positions. One DEI based image processing technique uses these points to extract the contrast components of refraction and apparent absorption from these image pairs.
The following paragraph describes of this technique for extracting the contrast components of refraction and apparent absorption from an image pair. When the analyzer crystal is set to an angle representing +/−½ DW for a given reflection and beam energy, the slope of the rocking curve is relatively consistent and can be represented as a two-term Taylor series approximation as represented by the following equation:
      R    ⁡          (                        θ          0                +                  Δθ          z                    )        =            R      ⁡              (                  θ          0                )              +                            ⅆ          R                          ⅆ          θ                    ⁢              (                  θ          0                )            ⁢                        Δθ          z                .            If the analyzer crystal is set to the low-angle side of the rocking curve (−½ DW), the resulting image intensity can be represented by the following equation:
      I    L    =                    I        R            ⁡              (                                            R              ⁡                              (                                  θ                  L                                )                                      +                                          ⅆ                R                                            ⅆ                θ                                              ⁢                      ❘                          θ              =                              θ                L                                              ⁢                      Δθ            z                          )              .  The recorded intensity for images acquired with the analyzer crystal set to the high-angle position (+½ DW) can be represented by the following equation:
      I    H    =                    I        R            ⁡              (                              R            ⁡                          (                              θ                H                            )                                +                                                    ⅆ                R                                            ⅆ                θ                                      ⁢                          (                              θ                H                            )                        ⁢                          Δθ              z                                      )              .  These equations can be solved for the changes in intensity due to apparent absorption (IR) and the refraction in angle observed in the z direction (ΔθZ) represented by the following equation:
            Δθ      Z        =                                        I            H                    ⁢                      R            ⁡                          (                              θ                L                            )                                      -                              I            L                    ⁢                      R            ⁡                          (                              θ                H                            )                                                                                      I              L                        ⁡                          (                                                ⅆ                  R                                                  ⅆ                  θ                                            )                                ⁢                      (                          θ              H                        )                          -                                            I              H                        ⁡                          (                                                ⅆ                  R                                                  ⅆ                  θ                                            )                                ⁢                      (                          θ              L                        )                                          I      R        =                                                                      I                L                            ⁡                              (                                                      ⅆ                    R                                                        ⅆ                    θ                                                  )                                      ⁢                          (                              θ                H                            )                                -                                                    I                H                            ⁡                              (                                                      ⅆ                    R                                                        ⅆ                    θ                                                  )                                      ⁢                          (                              θ                L                            )                                                                          R              ⁡                              (                                  θ                  L                                )                                      ⁢                          (                                                ⅆ                  R                                                  ⅆ                  θ                                            )                        ⁢                          (                              θ                H                            )                                -                                    R              ⁡                              (                                  θ                  H                                )                                      ⁢                          (                                                ⅆ                  R                                                  ⅆ                  θ                                            )                        ⁢                          (                              θ                L                            )                                          .      These equations can be applied to the high and low angle images on a pixel-by-pixel basis to separate the two contrast elements into what is known as a DEI apparent absorption and refraction image. However, it is important to note that each of the single point rocking curve images used to generate DEI apparent absorption and refraction images is useful.
Development of a clinical DEI imager may have significance for women's health and medical imaging in general for the following reasons: (1) DEI has been shown to produce very high contrast for the features that are most important to detection and characterization of breast cancer; (2) the physics of DEI allows for imaging at higher x-ray energies than used with absorption alone; and (3) the ability of DEI to generate contrast without the need of photons to be absorbed dramatically reduces ionization, and thus reduces the absorbed dose.
Further, screen-film mammography has been studied extensively for the last 40 years, and because of many large randomized screening trials, it is known to reduce breast cancer mortality by approximately 18-30%. The rate of breast cancer death in the last few years has begun to decline, likely due in part to the widespread use of this imaging test. However, standard screen-film mammography is neither perfectly sensitive nor highly specific. Dense breast tissue and diffuse involvement of the breast with tumor tends to reduce the sensitivity of screening mammography. For women with dense breasts, lesions that develop are difficult to see because their ability to absorb photons is not much greater than the surrounding adipose tissue, generating little contrast for visualization. Approximately 10-20% of breast cancers that are detected by self-examination or physical examination are not visible by screen-film mammography. In addition, when lesions are detected by mammography and biopsy, only 5-40% of lesions prove to be malignant. Furthermore, approximately 30% of breast cancers are visible in retrospect on prior mammograms.
Current DEI and DEI imaging processing techniques are based heavily on conventional imaging theory and rely, at least in part, on X-ray absorption for image generation. Thus, objects imaged using these techniques absorb radiation. Such radiation exposure is undesirable in applications for medical imaging given concerns of dose, and this reasoning places considerable engineering limitations that make clinical and industrial translation challenging. Thus, it is desirable to provide DEI and DEI techniques that produce high quality images and that rely less on absorption but produce images with equivalent diagnostic quality and feature visualization. In addition, it is desirable to reduce DEI imaging time, which can be affected by the significant reduction of beam flux in DEI monochromators.